A typical prior art apparatus for scanning electron microscopy is shown in FIG. 1. As shown therein, a sample 10 to be analyzed is fixed to a platen 12 inside a chamber 14 of a scanning electron microscope. The microscope emits a plurality of electrons 16 toward the sample 10, each at for example energy E.sub.0 electron volts. The platen 12 is in turn fixed to the body of the microscope, which is at ground potential, so that the platen 12 and sample 10 are nominally at ground potential.
Of the electrons which contact the sample 10, a number thereof scatter from the sample 10 with complete elasticity, i.e., each such electron maintains the energy E.sub.0 electron volts. The remaining electrons from the incident beam are inelastically scattered from the sample into continuum of energies, from 0 electron volts up to E.sub.0 electron volts, one for example being shown with an energy of E.sub.0 -.DELTA.E electron volts.
A phosphorus recording plate 18 is included within the chamber 14, also being held at ground potential, and as is well known, a diffraction pattern is defined on the recording plate 18 by the elastically scattered electrons reaching the plate 18 so that information about the crystallography of the sample 10 can be obtained. It is important to note that inelastically scattered electrons do not contribute to the desired diffraction pattern signal, but in fact obscure such a pattern by raising the level of background noise.
As pointed out in the article "Effect of Energy Filtering on Micro-Diffraction in the SEM", by James F. Mancuso et al., Proceedings Fifty-Second Annual Meeting Microscopy Society of America Twenty-Ninth Annual Meeting Microbeam Analysis Society, 31 Jul. 5 Aug. 1994, San Francisco Press, Inc., it is well known that providing an electron energy filter between the sample 10 and the recording plate 18 can decrease the background noise increasing the diffraction pattern visibility. The above-cited article describes various experiments aimed toward achieving this goal. The filter typically takes the form of a grid positioned between the sample 10 and the recording plate 18 and carrying a large negative electrostatic potential so as to repel electrons of lower energy, thus allowing only electrons with sufficient energy to pass through the filtering medium. As is well known, having the recording plate 18 very close to the sample 10 is necessary to insure that the recording plate 18 receives a large solid angle of electrons scattered from the sample 10, so that the recording plate 18 receives a substantial amount of information concerning the sample 10. It will be seen that this advantage is somewhat negated if a filter is placed between the sample 10 and the recording plate 18, forcing the sample 10 and recording plate 18 to be moved further apart.
Also, with the filter positioned between the sample 10 and recording plate 18, the field of view is blocked to an extent by the filter.
Also of general interest is the article "Energy-Filtering Transmission Electron Microscopy in Materials and Life Science", by L. Reimer, Proceedings Fifty-Second Annual Meeting Microscopy Society of America Twenty-Ninth Annual Meeting Microbeam Analysis Society, 31 Jul.-5 Aug. 1994, San Francisco Press, Inc.